its the "against-all-odds" thing going... people like to root for the underdog.... and the way it's sometimes portrayed makes you sympathize? especially if the characters are good lookinng... lol... but this is kinda for non-hentai anime...
for me, im against incest, cos its just wrong, plus in-breeding has really terrible consequences. but cousins, i accept that in some societies, this is permissible and completely normal...